1. Field of the Invention
The present invention relates generally to a method and apparatus for cell re-selection in a mobile terminal.
2. Description of the Related Art
At present, a Universal Mobile Telecommunications System (UMTS) terminal supporting both GSM and WCDMA at the same time has a higher priority for WCDMA than for GSM. Thus, the terminal performs cell re-selection or handover to WCDMA having a higher priority in an area simultaneously servicing two kinds of Radio Access Technologies (RATs). Handover for the terminal is implemented under the control of a network, but cell re-selection is determined and implemented by the terminal itself in an idle mode without the control of the network. Accordingly, in case where the terminal is located in a service area where there are both WCDMA and GSM cells, the terminal re-selects a WCDMA cell or a GSM cell satisfying a predetermined condition in idle mode.
FIG. 1 is a flowchart illustrating a conventional cell re-selection procedure in a mobile terminal. It is assumed that the terminal is in idle mode.
Referring to FIG. 1, in step 101, the terminal determines if a signal received from a serving WCDMA cell is below a threshold or weakened to an extent of making it difficult to maintain a service. In step 103, the terminal measures a received signal of a target GSM cell located in a service area and determines if a specific condition for re-selecting the target GSM cell is satisfied, when it is determined that the received signal of the serving WCDMA cell is below a threshold. In step 105, the terminal re-selects the target GSM cell and performs handover to the re-selected GSM cell, when the specific condition is satisfied.
In step 107, the terminal determines if a received signal level of the GSM cell is out of a range of a preset level. The preset level has a range of 0 to 7 or 8 to 14. In step 109, the terminal analyzes system information received from a network and identifies a target WCDMA cell located in a service area, when it is determined that the received signal level is out of the preset level range. In step 111, the terminal measures a received signal of the target WCDMA cell and determines if a cell re-selection condition of Table 1 for handover to the target WCDMA cell is satisfied.
Table 1 shows conditions for re-selecting a WCDMA cell in a terminal that is camping on to a GSM cell.
TABLE 1Conditions for handover from GSM cell to WCDMA cell (3GPP.05.08)Condition 1:Ec/No >= FDD_QminCondition 2:RSCP >= FDD_Qoffset + RLA_CCondition 3:RSCP >= FDD_Qoffset + RLA_n
Referring to Table 1, Ec/No and RSCP denote energy per chip to noise ratio and received signal code power of target WCDMA cell, respectively. RLA_C denotes Received Level Average (RLA) of serving GSM cell. RLA_n denotes RLA of the “n” number of neighbor GSM cells. FDD_Qmin denotes minimum threshold value for Ec/No for frequency division duplex (FDD) cell re-selection. FDD_Qoffset denotes offset between signal strength of GSM and WCDMA cells. FDD_Qoffset is received from the network. The terminal increases FDDQ_offset received from the network by 5 dB and determines if the conditions are satisfied, when attempting to perform handover to the WCDMA cell within 15 seconds after re-selecting a GSM cell.
In step 113, the terminal re-selects the target WCDMA cell and camps on the re-selected WCDMA cell when it is determined that the conditions of Table 1 are satisfied. After that, the terminal terminates the procedure according to the present invention.
As described above, in case where there is a target WCDMA cell in a service area when a terminal camps on to a GSM cell, the terminal measures RLAs of a serving GSM cell and target WCDMA and GSM cells located in the service area every constant period, determines if there is a target WCDMA cell satisfying the conditions of Table 1, and re-selects the target WCDMA cell and camps on the re-selected WCDMA cell when it is determined that there is the target WCDMA cell satisfying the conditions.
However, in case where the re-selected WCDMA cell barely satisfies the conditions of Table 1 and gets increasingly worse, the terminal again re-selects a GSM cell and performs handover to the re-selected GSM cell and again attempts a re-selection from the GSM cell to the WCDMA cell barely satisfying the conditions. As above, when the terminal re-selects the WCDMA cell satisfying the conditions of Table 1, a ping-pong effect can occur in which a cell re-selection is repeated between the GSM and WCDMA cells. Also, this causes a great increase of an amount of an electric current consumed by the terminal.